Heat exchanger apparatus

ABSTRACT

A heat exchanger apparatus includes at least one metal sheet with a corrugated surface, fabric covering at least a part of one surface of the at least one metal sheet to promote evaporation, and a wetting agent in the fabric to promote wetting of the fabric, the wetting agent also acting as an anti-microbial agent.

RELATED APPLICATION DATA

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/342,144, filed Apr. 15, 2019, which is a U.S. national phaseapplication based on and claiming priority benefit of internationalapplication no. PCT/EP2017/075939, filed Oct. 11, 2017, and whichclaimed priority to British national application no. GB 1617362.7, filedOct. 13, 2016. The entire contents of these prior filed applications arehereby incorporated by reference herein.

TECHNICAL FIELD

The invention relates to cooling systems relying on evaporative cooling,and more particularly to a heat exchanger apparatus, a cooling system,and methods of operation thereof.

BACKGROUND

It is known to provide a dew point air conditioner comprising a flatplate heat exchanger with complex heat exchanger sheets, fans, pumps,enclosure and control devices. The known heat exchanger surfacetypically does not have any treatment, leading to a poor waterdistribution and insufficient wetting, especially in the horizontaldirection, due to the effect of gravity on the water. This leads to apoor heat and mass transfer between dry and wet channel air streams ofthe heat exchanger. Furthermore, due to the poor water distribution andinsufficient wetting, the amount of circulating water within the airconditioner leads to a substantial power consumption by the pumps of theair conditioner.

The known flat plate heat exchanger makes inefficient use of the heatexchanging space, leading to a smaller heat transfer capacity. The knownheat exchanger uses air guides between the adjacent heat exchangersheets which increases air flow resistance between the sheets, leadingto a higher fan power consumption.

Problems with known arrangements include excessive power consumption,excessive water consumption, and, due to the presence of bacteria etc.,fouling of the heat exchanger in addition to the increased risks tohealth. The known flat plate heat exchanger may also have a reduced lifespan due to fouling and corrosion. It is known that some wetting agentscan act as an anti-microbial agent. Furthermore, the use of wettingagents for evaporative cooling is known. For example, LiCl is a knownmoisture absorbent and surfactant, which can also have antimicrobialproperties and Poly Vinyl Alcohol (PVA) is known for its use withcontact lenses and in dehumidifiers. However, known techniques foroperating air cooling apparatus do not involve the addition of LiCl toPVA for use in evaporative cooling to enhance the anti-microbialproperties.

US2005/0218535A1 discloses a method and apparatus for indirectevaporative air-cooling in which each plate of the apparatus is made oflaminate material having one sheet of wicking material for the wetzone(s) and another sheet of a water proof plastic material for the dryzone(s). The wick layer can be made of cellulose, polyester,polypropylene or fiberglass. One embodiment uses corrugated sheets, thecorrugations forming the guides for the flow of air and thus formingchannels. The channels are maintained by having the corrugations ofadjoining plates oriented such that they are not parallel (e.g. they areat right angles) and do not nest with the adjoining plate. The plasticlayer, and the cross-flow arrangement, does not assist in optimizingheat-transfer, and the use of the preferred wick material does notoptimize the maintenance of moistness.

US2013/0233005A1 discloses a method of controlling the operation of anevaporative air cooler where pads (i.e. corrugated paper media) of thecooler are intermittently wetted with an amount of water to absorb andretain during each wetting operation. Any free water within theevaporative media (pad) is allowed to flow back into a reservoir belowthe pad. Static pressure transducers on either side of the pads enableairflow velocity there through to be measured. The airflow is limited toa velocity so as not to entrain water in the airflow during the wettingoperation, and the velocity of the airflow is increased after eachintermittent wetting so as to raise the level of cooling output of thecooler between each intermittent wetting operation. The wetting sequencemay take up 10%-20% of the operating time. The corrugated paper mediadoes not assist in optimizing heat-transfer, and the disclosed wettingtechnique does not optimize the maintenance of moistness while reducingwater or power usage.

The known materials generally used for heat exchanger sheets in dewpoint air cooler apparatus are non-metallic materials (e.g. Kraft paper,plastic etc.). Such non-metallic materials have poor heat conduction.

It is broadly an object of the present invention to address one or moreof the above mentioned disadvantages of the previously known heatexchanger apparatus.

SUMMARY

What is required is a heat exchanger apparatus, which may reduce orminimize at least some of the above-mentioned problems.

According to a first aspect of the invention, there is provided a heatexchanger apparatus, comprising at least one metal sheet with acorrugated surface, fabric covering at least a part of one surface ofthe at least one metal sheet to promote evaporation, a wetting agent inthe fabric to promote wetting of the fabric, and which also acts as ananti-microbial agent.

Such an apparatus provides the advantage that the corrugations canreduce flow resistance due to the lack of impeding internal supports,the fabric and wetting agent can enhance evaporation rate and thewetting agent can prevent negative effects such as fouling, corrosion &bacteria growth, and these all synergistically enhance or optimizeflowrate and cooling efficiency of the heat exchanger apparatus. Inaddition, such irregular/corrugated heat exchanging sheets can increaseheat and mass transfer area by around 35%, leading to the samepercentage of increase in heat transfer rate.

Preferably, the heat exchanger apparatus has an axis of elongation, theaxis of elongation, in use, being arranged vertically (i.e.substantially vertically).

Preferably, the corrugated surface comprises corrugations over a firstportion of the one surface of the heat exchanger apparatus. In oneembodiment, the fabric covers a second portion of the one surface, thesecond portion overlapping the first portion. In another embodiment, thefabric covers a second portion of the one surface, the first portionbeing fully contained within the second portion. Advantageously, thefabric thus covers all of the corrugations, so that the fabric enhancesevaporation rate over the entire corrugated surface.

Preferably, the one surface of the heat exchanger apparatus comprises,outside the first portion, a third portion, the third portion beingflat. The third portion is a portion or part of the one surface that isflat or planar. This can enhance flow efficiency in the vicinity of thethird portion, i.e. near air inlets and outlets respectively.Preferably, in use, (i) the first portion is disposed on an upperportion of the heat exchanger apparatus and/or is contiguous with an airoutlet, and/or (ii) the third portion is disposed on a lower portionthereof and/or is contiguous with an air inlet. In this contextcontiguity means that an edge of the first portion, or an edge of thethird portion shares an edge or boundary with the air inlet or outlet asrequired.

Preferably, the one surface of the heat exchanger apparatus comprises,outside the first portion, a fourth portion, the fourth portion beingflat. Preferably, in use, the fourth portion is disposed on an uppermostportion of the heat exchanger apparatus and/or is contiguous with an airoutlet.

Preferably, the corrugated surface has in cross-section a profile of aperiodic waveform. The waveform may be quasi sinusoidal or triangular.This configuration enables abutment of adjacent sheets at the edges(peaks) formed by the corrugations, or near to those peaks, to formeffective airflow channels. This configuration beneficially omitschannel supporters (or so-called ‘air guides’) from the heat exchangerstack, leading to around 55% reduction in air flow resistance and thesame percentage of fan power saving.

Preferably, the periodic waveform has a peak-to-peak distance of (i)10-13 mm, (ii) 11-12 mm or (iii) 11.6 mm Preferably, the periodicwaveform has an amplitude of (i) 2-3 mm, (ii) 2.2-2.8 mm or (iii) 2.5 mmPreferably, corrugations intersect the plane of the heat exchangerapparatus at an angle to that plane of (i) 46-54 degrees, (ii) 48-52degrees, or (iii) 50 degrees. Preferably, the corrugated surface has adimension, in the direction transverse to the direction of extension ofcorrugations therein, of (i) 300-400 mm, (ii) 325-375 mm, or (iii) 348mm.

Preferably, the fabric is a synthetic moisture-wicking fabric.Preferably, the fabric is formed of polyester. Preferably, the fabric isformed of textile fabric. The inventors have discovered that the use offabric made from a material such as polyester fibre can significantlyimprove the wet-ability and water diffusivity across the surface of thefabric, thus enhancing its heat transfer performance.

Preferably, the wetting agent is a haloid and polyvinyl surfactant.Preferably the wetting agent comprises Polyvinyl Pyrollidone (PVP) orPolyvinyl-Alcohol (PVA). Preferably the wetting agent comprises a saltcompound.

Preferably, the wetting agent is a LiCl/Polyvinyl-Alcohol (PVA)solution. The inventors have discovered that the use of theLiCl/Polyvinyl-Alcohol coating solution on the wet surface of the heatexchanging sheets can enhance the heat exchanger sheets' wateradsorption capacity, and prevent a negative effect (e.g. fouling,corrosion & bacteria growth) on the surface. This enhances efficiency ofoperation and can prevent adverse health effects.

Preferably, the metal sheet is formed of aluminium. This assists inimproving/maximising heat transfer from dry channels to wet channels.Preferably, a marine adhesive is used in bonding the fabric to thealuminium heat exchanger sheets, advantageously creating a strong bondand long life-span for the heat exchanging sheets.

Preferably, some or all of the corrugations in the corrugated surfacehave one or more through holes at or near one end thereof. This assistsin separating the flow into exit air and returning/exhaust air.Preferably, in use, the through holes are disposed at or near anuppermost end of the heat exchanger apparatus. This further assists inseparating the flow into exit air and returning/exhaust air, i.e.shortly before the outlet of cooled working air.

Preferably, a heat exchanger apparatus according to the writtendescription and claims is provided, comprising a plurality of metalsheets, wherein, when two or more heat exchangers are placed together,corrugated surfaces of adjacent metal sheets define channels to promoteair flow.

Preferably, a water supply system is included, the water supply systembeing configured for supplying water to the fabric of the heat exchangerapparatus for evaporative cooling. Preferably, a controller includedthat is coupled to the water supply system, the controller beingconfigured for intermittently operating the water supply system for theapplication of water to the fabric to maintain the fabric in a moistcondition. Preferably, the water supply system is configured torecirculate water to the heat exchanger apparatus from a water sump.

According to a second aspect of the invention, there is provided acooling system comprising: a heat exchanger apparatus according to thewritten description and claims; a water supply system, configured forsupplying water to the fabric of the heat exchanger apparatus forevaporative cooling; a controller, coupled to the water supply system,the controller being configured for intermittently operating the watersupply system for the application of water to the fabric to maintain thefabric in a moist condition.

According to a third aspect of the invention, there is provided acooling system comprising: a heat exchanger apparatus according to thewritten description and claims; a water supply system, configured forsupplying water to the fabric of the heat exchanger apparatus forevaporative cooling; wherein the water supply system is configured tore-circulate water to the heat exchanger apparatus from a water sump.

According to a fourth aspect of the invention, there is provided amethod of operating a cooling system, the method including: providing aheat exchanger apparatus according to the written description andclaims; providing a water supply system, configured for supplying waterto the fabric of the heat ex-changer apparatus for evaporative cooling;providing a controller, coupled to the water supply system; and usingthe controller, intermittently operating the water supply system for theapplication of water to the fabric to maintain the fabric in a moistcondition.

An advantage of this aspect is that the intermittently running of thewater circulation keeps the wet surface of the heat exchanging sheet ina saturation state while preventing formation of a water film on thesurface, which would reduce the evaporation of water.

According to a fifth aspect of the invention, there is provided a methodof operating a cooling system, the method including: providing a heatexchanger apparatus according to the written description and claims;providing a water supply system, configured for supplying water to thefabric of the heat exchanger apparatus for evaporative cooling; usingthe water supply system, recirculating water from a water sump.

An advantage of this aspect is that, while the water supply is mainlydelivered by tap water, a circulation pump is only activated when thebottom tank is full; this therefore minimises the pump operational timeand its power consumption.

According to an alternative characterisation of the invention there isprovided a heat exchanger apparatus, comprising at least one metal sheetwith a first part having a corrugated surface, second part having a flatsurface, and a third part having a flat surface, the second and thirdparts on either side of the first part, a fabric covering at least aportion of one surface of the metal sheet to promote evaporation, awetting/anti-microbial agent in the fabric to promote wetting of thefabric and inhibit microbial growth, wherein the fabric covers at leasta portion of the first part, and one or both of the second part and thethird part of the one surface.

In use, the heat exchanger apparatus is disposed with the metal sheetbeing vertical such that the second part is contiguous with an airoutlet, and the third part is contiguous with an air inlet.

Any preferred or optional features of one aspect or characterization ofthe invention may be a preferred or optional feature of other aspects orcharacterizations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the invention will be apparent from the followingdescription of preferred embodiments shown by way of example only withreference to the accompanying drawings, in which;

FIG. 1 shows a perspective side view of a heat exchanger sheet accordingto an embodiment of the invention;

FIG. 2 shows a perspective side view of a stack of heat exchanger sheetsshown in FIG. 1 ;

FIG. 3 is a schematic perspective view of four sheets of the apparatusof FIG. 2 ;

FIG. 4 is a cross section across the heat exchanger sheet shown in FIG.1 ;

-   -   and

FIG. 5 shows steps of a method according to an embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 shows a perspective side view of a heat exchanger sheet accordingto an embodiment of the invention, generally designated 10. In FIG. 1the heat exchanger sheet is shown in its normal orientation in use. Theheat exchanger sheet 10 is of series 5005 aluminium. The heat exchangersheet 10 is corrugated over an upper region 14. The series 5005aluminium used to form the heat exchanger sheet 10 provides suitableductility to form the corrugations, and also provides the requiredthermal conductivity. An upper region 15 of the heat exchanger sheet 10is planar. A lower region 16 of the heat exchanger sheet 10 is planar.Also shown is that the transition 17 from each of the upper region 15and the lower region 16 to the corrugations is smooth. The heatexchanger sheet 10 is 360 mm wide, and 1000 mm long. The corrugatedregion of the heat exchanger sheet 10 is 860 mm in length. The planerregion 16 is 115 mm in length, and the planer region 15 is 20 mm inlength. As shown in FIG. 1 the heat exchanger sheet 10 is elongate andan axis of elongation 19 thereof is indicated, which is a direction ofthe heat exchanger sheet 10 with the greatest dimension. In use the axisof elongation 19 is arranged vertically (i.e. substantially vertically).The axis of elongation 19 is also the direction in which the channelsrun and is also the direction in which fluid flows as discussed below.

A fabric is glued to one side of the heat exchanger sheet 10 as shown bythe dashed outline 12. The fabric is preferably formed of a materialexhibiting the following, or substantially similar, properties in a testof the capillary wick effect (i.e. water suction height), when thematerial is suspended with its lower end in water:

Wicking height (cm) (a) 1.0 1.5 3.1 3.6 5.2 6.2 (b) 4.9 6.3 9.0 11.014.2 16.5

More preferably, the fabric is formed of Coolmax-2 ((b) above),available from Invista. The row (a) above is for craft paper and isincluded as a control to show the advantages of using Coolmax-2.

The fabric is preferably formed of a material exhibiting the following,or substantially similar, properties in a test of water diffusivity andevaporation rate, when the material is glued to the aluminium sheet, haswater drops applied to it and is observed from above:

Drop diffusion time Wetted area (cm²) Evaporation time (min) (a) 7′30″ 9′20″ 5.6 57.3 24′10″ 46′40″ (b)   57″ 11′45″ 3.4 64.4 13′12  29′10″

More preferably, the fabric is formed of Coolmax-2 ((b) above),available from Invista. The row (a) above is for craft paper and isincluded as a control to show the advantages of using Coolmax-2. It willbe appreciated that the Coolmax fabric is a polyester material, which issynthetic and generally resistant to degradation, which might be termeda synthetic moisture-wicking fabric, e.g. a woven fabric that issynthetic and moisture-wicking.

The operation to apply the fabric to the aluminium sheet involvesgrinding of the aluminium surface to abrade it, applying a specificSikaflex®-291i Marine Adhesive on the rough surface, and spreading thefabric (preferably Coolmax-2 fibre) on the glued surface. It has beenfound by the inventors that an excellent binding up effect is thuscreated, leading to the enhanced heat transfer between the dry and wetsides of the sheets 10, and increased water evaporation on the wetsurface of the sheets 10. Such an adhesive provides a suitable heattransfer between the aluminium heat exchanger sheet 10 and the fabric12. In one arrangement for manufacture of the heat exchanger sheet 10, ahotplate (not shown) is preheated to a designated temperature andallowed to stabilise for about 30 minutes, the heat exchanger sheet 10is then placed on the hotplate for about 5 minutes, the adhesive is thenspread over the heat exchanger sheet 10, and then the fabric 12 is puton to the adhesive on top of the heat exchanger sheet 10. Pressure isthen applied on the top of the fabric 12 (for example by placing a masshaving a suitable profile onto the fabric 12), the hotplate is thenallowed to cool until the adhesive is set. Such a way of applying thefabric 12 to the heat exchanger sheet 10 may be termed a hot pressuremethod.

To complete preparation of the fabric, a wetting agent, preferably ahaloid polyvinyl surfactant solution, e.g. LiCl/Polyvinyl-Alcohol wateraffinity solution, is applied to the fabric. It has been found by theinventors that an enhanced water diffusivity effect is achieved and,over long-term operation, the surface had no fouling, corrosion, as wellas no bacterial growth. The LiCl/Polyvinyl-Alcohol solutions act as awater affinity solution (i.e. a wetting agent), and also acts as ananti-microbial agent. Alternatively, Polyvinyl Pyrollidone (PVP) may beused instead of the Polyvinyl-Alcohol (PVA) wetting agent. Furthermore,the haloid and polyvinyl surfactant solution may alternatively be termeda polymeric wetting agent, which also acts as an anti-microbial agent.In general terms the wetting agent/anti-microbial agent is a mixture ofa salt and a wetting agent, for example, a combination of a lithium saltand a wetting agent.

Returning to FIG. 1 , in each heat exchanger sheet 10, two rows ofthrough holes 18 are provided at an upper end of the corrugations. Inone arrangement, up to four rows of through holes 18 may be provided.The through holes 18 are in a side of each raised portion of eachcorrugation. As will be described in more detail below, duringoperation, the intake air is drawn into a heat exchanger apparatusincorporating a stack of the heat exchanger sheets 10. This part of theair travels along dry channels formed by the stack. At the end of thedry channels, the air is divided into two parts: one is delivered to theroom space for space cooling while the other is diverted to the adjacentwet channels, via through holes 18.

FIG. 2 shows a perspective side view of a stack of heat exchanger sheets10 shown in FIG. 1 , generally designated 20. In FIG. 2 like features tothe arrangements of FIG. 1 are shown with like reference numerals. InFIG. 2 the heat exchanger sheets 10 are placed next to each other sothat the aluminium sides of adjacent heat exchanger sheets 10 are facingeach other, and so that the fabric sides of adjacent heat exchangersheets 10 are facing each other. The corrugations of adjacent sheetsprovide vertical channels such that there are “wet channels” havingfabric sides, and “dry channels” having aluminium sides. In thisembodiment the gap between two heat exchanger sheets 10 to form a wet ordry channel is at its maximum 5 mm. The vertical channels comprisetrapezoid or rhomboid flow channels for the “wet” and “dry” air. Thesurfaces of the adjacent sheets 10 do not require supporters to formthese trapezoid or rhomboid flow channels, which avoids a restriction tothe air flow. Such an arrangement of adjacent sheets 10 stops moisturetransfer between adjacent channels and means that only heat transfer canhappen between adjacent “wet” or “dry” air channels, also which providesan overall improved operation of the heat exchanger sheets 10.

In FIG. 2 the ambient intake air (either from outside of the room ormixture of outside and inside of the room air, received via an inlet airfilter) is shown at 22, the cool product outlet air is shown at 24, andthe working air and water discharge are shown at 26. Water is input atthe top of each “wet channel”. It will be appreciated that whereas thestack of heat exchanger sheets are shown at 28, in practice the sideswould be closed so that air or water does not escape. In practice theremight be up to 100 heat exchanger sheets 10 in a stack.

It will be appreciated that the flat regions 15 and 16 of each heatexchanger sheet 10 comprise inlet and outlet regions respectively. Suchflat parts of the heat exchanger sheets 10 permit ready air and waterdistribution within the channels of the stack of heat exchanger sheets20.

FIG. 3 is a schematic perspective view of four sheets of the apparatus20 of FIG. 2 , generally designated 30. In FIG. 3 like features to thearrangements of FIG. 2 are shown with like reference numerals. In FIG. 3four heat exchanger sheets 10 are shown with the corrugations omittedfor clarity. Surfaces of two of the heat exchanger sheets 10 havingfabric on them are shown at 32 (i.e. the fabric 32 is on one side ofeach sheet 10). Aluminium surfaces of two of the heat exchanger sheets10 are shown at 34 (i.e. the aluminium surface 34 is one side of eachsheet 10). With such an arrangement the fabric 32 of a surface of onesheet 10 faces the fabric 32 of a surface of another sheet 10.Similarly, the aluminium surface 34 of one sheet 10 faces the aluminiumsurface 34 of another sheet 10. Also shown is a water reservoir 36having a water level 38. The water reservoir is filled from a supply ofwater 40, which may be tap water. A pump 42 transfers water from thewater reservoir 36 along conduit 44 to two water ducts 46 at the top ofthe two wet channels. Water 46 from the water ducts 46 wets the fabric32 on each heat exchanger sheet 10, and the fibres of the fabric withthe wetting agent help to spread moisture on the fabric 32 and enlargethe wetted area, which promotes evaporation. A valve device (not shown)may be provided so that the supply of water 30 is shut off when thereservoir water is at a certain level.

The ambient intake air 22 is drawn into a dry channel by a fan (notshown). The intake air is then split into two streams whereby one stream50 passes into the through holes 18 and the other stream exits as thecool product outlet air 24. The stream 50 then enters the two wetchannels as shown at 52 where it causes water to evaporate from thefabric. The stream 50 also absorbs the heat transported from the drychannel and receives the moisture evaporated from the wet channelsurface, i.e. the fabric 32. It will be understood that the fabric 32promotes high evaporation due to the fibres and wetting agent thereof.It will be appreciated that a top of each wet chamber is closed so thatthe air cannot escape from the top of each wet channel and is forceddownwards as shown at 52. The working air and water discharge are shownat 26. Such an arrangement may be termed a counter flow heat exchangerwhereby inlet ambient air 22 passes in one direction (i.e. upwards) onone side of a sheet 10 and working air and water 26 pass in anotherdirection (i.e. downwards) on another side of the sheet 10. It will beunderstood that the inlet ambient air 22 passes upwards in a “drychannel”, and the working air and water 26 pass downwards in a “wetchannel”. With such an arrangement each of four corrugated heatexchanger sheets 10 works as a heat transfer plate with an increasedheat transfer area. The air discharged at 26 is hot and humid. It willbe appreciated that the air 22 travels along the dry channels of thestack where it loses heat and becomes colder due to the establishedtemperature difference between the dry and wet sides of the sheets 10caused by the water evaporation on the wet surface. In effect thecorrugated region provides an increased heat and mass transfer areabetween the air 22 and the air 26.

The four sheets 10 are only a part of the stack of heat exchanger sheets10, and the four sheets of FIG. 3 are a small working unit of theapparatus. In practice the four-sheet unit would be repeated severaltimes in a stack of heat exchanger sheets 10. It will also beappreciated that a controller (not shown) is coupled to the water supplysystem and is configured for intermittently operating the water supplysystem for the application of water to the fabric to maintain the fabricin a moist condition. The controller is also configured to recirculatewater to the heat exchanger apparatus from the water reservoir 36, whichmay also be termed a water sump.

FIG. 4 is a cross section across the heat exchanger sheet shown in FIG.1 . As seen in FIG. 4 , the corrugated surface has in cross-section aprofile of a periodic waveform. Such a waveform may be quasi sinusoidal,triangular or other. An irregular or regular corrugated surface may beused.

The corrugated surface has a horizontal dimension W, in the directiontransverse to the direction of extension of corrugations therein, of300-400 mm, 325-375 mm, or, in a preferred embodiment, 348 mm. In anyevent, the heat exchanger sheets 10 may have side strips 59, i.e. at theedges of the sheet 10, that are 6 mm wide, thereby facilitatingmounting/fixing of the sheets in a stack.

The corrugations are chosen to be of a particular dimension to promoteair flow. Thus, in terms of the aforementioned periodic waveform, it mayhave a peak-to-peak distance T (i.e. wavelength) of 10-13 mm, 11-12 mm,or in a preferred embodiment 11.6 mm. In addition, the periodic waveformmay have an amplitude 2A of 4-6 mm, 4.5-5.5 mm or, in a preferredembodiment, 5 mm Thus, in this embodiment the corrugations are 2.5 mmdeep and 5.8 mm wide from the plane of the sheet 10. It has been foundby the inventors that corrugations of these dimensions produce channelsthat are particularly effective in enhancing heat exchanger efficiency.FIG. 4 also shows that the corrugations intersect the plane of the heatexchanger apparatus at an angle to that plane of 50 degrees, and theangle may be between 46-54 degrees, or in a preferred embodiment 48-52degrees.

It will be understood from FIG. 4 that the plane of the heat exchangersheet 10 is shown at 57, and that the flat regions 15, 16 are coplanarwith the plane 57. Such an arrangement facilitates the water distributorand the inlet/outlet of the air flow through the stack of heat exchangersheets 10.

FIG. 5 shows steps of a method according to an embodiment of theinvention, generally designated 60. It will be appreciated that thesteps may be performed in a different order and may not necessarily beperformed in the order shown in FIG. 5 .

Initially, at step 62, a plurality of heat exchanger sheets 10 areprovided; this may involve tens or hundreds of heat exchanger sheets 10.Then, the heat exchanger sheets 10 are formed into a stack or heatexchanger apparatus at step 64. As part of a cooling system ashereinbefore described, a water supply system (including sump, conduitsetc.) is provided, at step 66. Thereafter, the cooling system isoperated at 68.

As part of the latter step, the method 60 may comprise the step 70and/or the step 72. Step 70 comprises receiving water from the stackinto the water sump and recirculating water back to the stack from thewater sump. It has been found by the inventors that with this smallamount of water supply, the water can be directly provided from a watertap, while the circulation pump need only be activated when the bottomwater tank is full. This can significantly reduce the operational timeof the pump and thus, minimise its power consumption.

Step 72 comprises intermittently operating the water supply system forthe application of water to the fabric. It has further been found by theinventors that water supply towards the wet surfaces of the heatexchanging sheets 10 should not be run continuously; instead, it shouldbe run intermittently, for example for 20 seconds during each tenminutes of operation. This method can create a saturated wet surfaceover the operational period, while preventing the formation of a waterfilm above the wet surface, which can cause a negative effect upon thewater evaporation.

Combination of the above features has led to an increase in the coolingsystem's energy efficiency (COP) by around 80 to 100%, which means thatthe new heat exchanger apparatus can achieve around 40 to 50% savings inenergy use compared to the existing best performance dew point(evaporative) cooling technology with the same amount of cooling output.This will contribute to global energy savings and carbon reductiontargets.

The above description describes the heat exchanger sheet 10 beingcorrugated over an upper region 14 thereof, the lower portion 16 beingsubstantially flat and the portion 15 above the upper region 14 alsobeing substantially flat. In an alternative arrangement the heatexchanger sheet 10 may be corrugated over substantially the entiresurface thereof. It will be understood that the area of the corrugatedregion 14 relative to the area of the heat exchanger sheet 10, or thearea of the flat regions 15, 16 may depend on the configuration of theheat exchanger apparatus, and the skilled person will know therequirements for determining the area of the corrugated region 14 and/orflat regions 15, 16.

In the foregoing description various portions of the heat exchangersheet 10 are shown, for example, the portion 14 with corrugations, theportion 12 with fabric, the lower portion 16 that is flat, and the upperportion 15 that is flat. It will be understood that the area of theportions 12, 14, 15, 16 relative to one another and/or to relative tothe area of the heat exchanger sheet 10 may depend on the configurationof the heat exchanger apparatus, and the skilled person will know therequirements for determining the area of the portions 12, 14, 15, 16. Inone arrangement the corrugated surface comprises corrugations over afirst portion 14 of the one surface of the heat exchanger sheet 10, andthe fabric covers at least a second portion 21 (see FIG. 1 ) of the onesurface, such that the second portion 21 at least overlaps the firstportion 14. It will be appreciated that the second portion 21 shown inFIG. 2 is for illustrative purposes only, and the amount of overlapbetween the first portion 14 and the second portion 21 may be largerthan shown. In another arrangement the corrugated surface comprisescorrugations over a first portion 14 of the one surface of the heatexchanger sheet 10, and the fabric covers at least a second portion 12of the one surface, the first portion 14 being fully contained withinthe second portion 12 (see FIG. 1 ).

What is claimed is:
 1. A heat exchanger apparatus comprising: at leastone metal sheet with a corrugated surface; fabric covering at least apart of one surface of the at least one metal sheet to promoteevaporation; and a wetting agent in the fabric to promote wetting of thefabric, the wetting agent also acting as an anti-microbial agent.
 2. Aheat exchanger apparatus according to claim 1, wherein the corrugatedsurface includes corrugations over a first portion of the one surface ofthe heat exchanger apparatus.
 3. A heat exchanger apparatus according toclaim 2, wherein (i) the fabric covers a second portion of the onesurface, the second portion overlapping the first portion, or (ii) thefabric covers a second portion of the one surface, the first portionbeing fully contained within the second portion.
 4. A heat exchangerapparatus according to claim 3, wherein the one surface of the heatexchanger apparatus includes, outside the first portion, a thirdportion, the third portion being flat, wherein the heat exchangerapparatus has an axis of elongation, the axis of elongation, beingarranged vertically in use of the heat exchanger apparatus, and wherein,in use of the heat exchanger, (i) the first portion is disposed on anupper portion of the heat exchanger apparatus and/or is contiguous withan air outlet, and/or (ii) the third portion is disposed on a lowerportion of the heat exchanger apparatus and/or is contiguous with an airinlet.
 5. A heat exchanger apparatus according to claim 4, wherein theone surface of the heat exchanger apparatus includes, outside the firstportion, a fourth portion, the fourth portion being flat, and wherein,in use of the heat exchanger apparatus, the fourth portion is disposedon an uppermost portion of the heat exchanger apparatus and/or iscontiguous with an air outlet.
 6. A heat exchanger apparatus accordingto claim 2, wherein some or all of the corrugations in the corrugatedsurface have one or more through holes at or near one end thereof, andwherein, in use of the heat exchanger apparatus, the through holes aredisposed at or near an uppermost end of the heat exchanger apparatus. 7.A heat exchanger apparatus according to claim 1, wherein the corrugatedsurface has, in cross-section, a profile of a periodic waveform, andwherein the periodic waveform has a peak-to-peak distance of (i) 10 to13 mm, (ii) 11 to 12 mm or (iii) 11.6 mm, and/or wherein the periodicwaveform has an amplitude of (i) 2 to 3 mm, (ii) 2.2 to 2.8 mm or (iii)2.5 mm, and/or wherein corrugations intersect a plane of the heatexchanger apparatus at an angle to the plane of (i) 46 to 54 degrees,(ii) 48 to 52 degrees, or (iii) 50 degrees.
 8. A heat exchangerapparatus according to claim 1, wherein the fabric is a syntheticmoisture-wicking fabric, and/or wherein the fabric is formed of (i) apolyester or (ii) a textile fabric.
 9. A heat exchanger apparatusaccording to claim 1, wherein the wetting agent comprises a haloid and apolyvinyl surfactant.
 10. A heat exchanger apparatus according to claim9, wherein the wetting agent comprises polyvinyl pyrollidone (PVP) orpolyvinyl-alcohol (PVA).
 11. A heat exchanger apparatus according toclaim 9, wherein the wetting agent comprises a salt compound.
 12. A heatexchanger apparatus according to claim 11, wherein the wetting agent isa lithium chloride (LiCl)/polyvinyl-alcohol (PVA) solution.
 13. A heatexchanger apparatus according to claim 1, wherein the at least one metalsheet comprises a plurality of the metal sheets, and wherein two or moreof the plurality of metal sheets are placed together such that thecorrugated surfaces of adjacent metal sheets define channels to promoteair flow.
 14. A heat exchanger apparatus according to claim 1, furthercomprising: a water supply system, the water supply system (i)configured to supply water to the fabric of the heat exchanger apparatusfor evaporative cooling, and including a controller coupled to the watersupply system, the controller configured to intermittently operate thewater supply system to apply water to the fabric to maintain the fabricin a moist condition while preventing formation of a water film, or (ii)configured to supply water to the fabric of the heat exchanger apparatusfor evaporative cooling, wherein the water supply system is configuredto recirculate water to the heat exchanger apparatus from a water sump.15. A heat exchanger apparatus according to claim 1, wherein the heatexchanger apparatus has an axis of elongation, the axis of elongationbeing arranged vertically in use of the heat exchanger apparatus.
 16. Aheat exchanger apparatus according to claim 1, wherein the at least onemetal sheet is formed of aluminium.
 17. A method for operating a coolingsystem, the method comprising: providing a heat exchanger apparatus, theheat exchanger apparatus including at least one metal sheet with acorrugated surface, fabric covering at least a part of one surface ofthe at least one metal sheet to promote evaporation, and a wetting agentin the fabric to promote wetting of the fabric, the wetting agent alsoacting as an anti-microbial agent; providing a water supply systemconfigured to supply water to the fabric of the heat exchanger apparatusfor evaporative cooling; and (i) providing a controller coupled to thewater supply system, the controller configured to intermittently operatethe water supply system to apply water to the fabric to maintain thefabric in a moist condition while preventing formation of a water film;or (ii) recirculating water from a water sump using the water supplysystem.
 18. A heat exchanger apparatus comprising: a plurality of metalsheets, each metal sheet of the plurality of metal sheets having acorrugated surface with corrugations, the corrugated surfaces ofadjacent metal sheets of the plurality of metal sheets defining airflowchannels to promote air flow; fabric covering at least a part of onesurface of each metal sheet to promote evaporation; and a wetting agentin the fabric to promote wetting of the fabric, the wetting agent alsoacting as an anti-microbial agent, wherein the airflow channels aredefined between the adjacent metal sheets by abutment of the adjacentmetal sheets substantially at peaks formed by the corrugations, andwherein, among the adjacent metal sheets, the fabric of a surface of onemetal sheet faces the fabric of a surface of an adjacent metal sheet,and the metal surface of one metal sheet faces the metal surface of anadjacent metal sheet.
 19. A method for operating a heat exchangerapparatus, the method comprising: providing a heat exchanger apparatushaving (i) a plurality of metal sheets, each with a corrugated surfacehaving corrugations, (ii) airflow channels defined between and byabutment of adjacent metal sheets of the plurality of metal sheetssubstantially at peaks formed by the corrugations to promote air flow,(iii) fabric covering at least a part of one surface of each of theplurality of metal sheets to promote evaporation such that the fabric ofa surface of one metal sheet faces the fabric of a surface of anadjacent metal sheet and the metal surface of one metal sheet faces themetal surface of an adjacent metal sheet, and (iv) a wetting agent inthe fabric to promote wetting of the fabric, the wetting agent alsoacting as an anti-microbial agent, the method comprising: providing awater supply system configured to supply water to the fabric of the heatexchanger apparatus for evaporative cooling; and (i) providing acontroller coupled to the water supply system, the controller configuredto intermittently operate the water supply system to apply water to thefabric to maintain the fabric in a moist condition while preventingformation of a water film; or (ii) recirculating water from a water sumpusing the water supply system.